Hitherto, electrically insulating tapes obtained by applying a tack-adhesive to one surface of a polyethylene film or a polyvinyl chloride film, etc., and cutting such into a suitable width have been used as electrically insulating materials for coating electrical wires or splicing cables, etc.
Recently, improvement of various types of properties including heat resistance of the electrically insulating materials has been further required. However, this requirement cannot often be met by the abovedescribed electrically insulating tapes.
Hence, in order to meet this requirement, electrically insulating materials comprising a polyimide film to which a silicone resin adhesive is applied or a thin layer of tetrafluoroethylene-hexafluoropropylene copolymer (referred to as FEP, hereinafter) is provided have been used. These electrically insulating materials cormprising a polyimide film as a main component have improved heat resistance as compared with the abovedescribed electrically insulating tapes. However, since the silicone resin adhesive of FEP has poorer heat resistance than the polyimide resin, use of them is restricted to use at a temperature lower than their heat resistant temperature. Accordingly, they are not satisfactory, yet, because the excellent heat resistance of the polyimide resin itself cannot be utilized.